With social demands to save energy and resources in recent years, the need for durable tires has led to the need for rubber materials excellent in fracture resistance, wear resistance, and crack growth resistance. Natural rubber is known as rubber excellent in these properties. Given the high price of natural rubber, however, it is necessary to develop synthetic rubber that is as durable as natural rubber.
In order to bring the properties of synthetic polyisoprene closer to those of natural rubber for improved durability, efforts have been conventionally made to improve elongational crystallinity by making synthetic polyisoprene high-cis (for example, see Patent Literatures (PTL) 1 to 3). Although the durability of synthetic polyisoprene is improved in this way, a large amount of catalyst is necessary to obtain a desired amount of polyisoprene, which causes high residual catalyst content in synthetic polyisoprene. There is thus a problem in that synthetic polyisoprene is less durable than natural rubber under high severity conditions.
It has been found difficult to efficiently produce a high molecular weight polymer for a polymer having an isoprene skeleton, as compared with a polymer composed of another monomer. This is likely to be the cause of lower durability under high severity conditions. Although there are known techniques of modifying chain ends with tin tetrachloride or titanium tetrachloride during polymer synthesis to increase the molecular weight for improved durability, such techniques involve much gelation and so have a problem of actually causing a decrease in durability.